The invention relates to a method for the sinter coating of a workpiece and a device suitable for carrying out the method.
Methods for producing protective coatings on metal surfaces, especially wire goods and small metal parts, by sintering-on plastic powder have been known for a long time and are in use. For carrying out such methods, suitable plastic powders are supplied, for example, by DEGUSSA AG, Marl under the trade name VESTOSINT.
The sinter coating of a workpiece conventionally takes place by first heating the workpiece to a temperature above the fusion temperature of the material to be sintered-on and then bringing the workpiece in contact with the material, generally in powder form. Contact takes place at ambient temperatures which must necessarily lie below the fusion temperature of the sinter material so that the workpiece loses heat during contact with the sinter material and finally falls below the fusion temperature of the sinter material whereby the sinter process comes to a standstill. The thickness of the layer deposited up till then on the workpiece is proportional to the time interval between the beginning of contact with the sinter material and the time at which the temperature falls below its fusion temperature. If the workpiece to be coated has a small material thickness, the cooling takes place more rapidly than in the case of a workpiece having a greater material thickness so that in order to achieve uniform layer thicknesses on workpieces having different material thicknesses, the temperatures to which the workpieces are heated before they are brought in contact with the sinter material must be different. In the case of simply shaped workpieces having a homogeneous material composition and uniform wall thickness, sinter coatings having a desired coating thickness can thus be achieved by a suitable choice of temperature at which workpieces are brought in contact with the sinter material.
In the case of a workpiece having a non-uniform wall thickness or inhomogeneous material composition, in general terms workpieces comprising sections having different surface-related heat capacity, this results in the problem that the sinter layers which are deposited on a section of higher surface-related heat capacity before this is cooled below the fusion temperature of the sinter material, are larger than those in a section having lower surface-related heat capacity. It is therefore difficult to provide such workpieces with a coating of uniform thickness. If a minimum layer thickness must be achieved on the sections having low surface-related heat capacity, it must be accepted that the resulting layer on other sections will be thicker. This not only results in undesirable increased costs because of the unnecessary consumption of sinter material but the different layer thicknesses also increase the probability of defects of the sinter layer which impair their protective effect for the workpiece located thereunder.
Rapid heating methods have been proposed to solve this problem wherein the heating of the workpiece is interrupted before workpiece has reached a homogeneous temperature distribution. This has the result that when brought in contact with the sinter material, sections of the workpiece having a low surface-related heat capacity have a higher temperature than those having a low surface-related heat capacity so that the time intervals before cooling below the fusion temperature and thus the resulting layer thicknesses for both sections become approximately the same. In principle it should be assumed that with such a method, by suitably selecting the heating conditions, i.e. the final temperature which would be established on a workpiece if it were continuously exposed to the rapid heating conditions and the time interval in which the workpiece is exposed to the rapid heating, temperature differences between sections of different heat capacity can be adjusted within certain upper limits and can be optimised to the same deposition layer thicknesses. However, it has been found in tests that no satisfactory layer qualities could be achieved in this way and that in particular in transition zones between sections having different surface-related heat capacities, there was a strong tendency towards layer defects.